A. Field of the invention
This invention constitutes a bridging of two commercially important fields in modern medical technology, namely magnetic resonance imaging (MRI), and extracorporeal shock wave lithotripsy (ESWL) to open up a new domain of nonsurgical treatments. The invention is the use of the static magnetic field of an MRI device for purposes other than imaging, specifically as a component of electromagnetic transducers and the use of the MRI device for automated control of procedures. The transducers may be used for the generation of shock waves for extracorporeal lithotripsy or for other applications of shock wave therapy or, in general for producing motion which may be useful in medical applications.
Related Art
Destroying various targets inside a human body with shock waves requires a specially designed instrument which is able to:
a) locate the target inside the human body; PA1 b) generate a special acoustic pressure wave; PA1 c) focus the acoustic pressure wave onto the target.
In the field of Extracorporeal Shock Wave Lithotripsy (ESWL) the usual targets are renal or gall-stones. These targets first must be located and positioned in the focal area of the shock wave generator. Then, by applying a set of powerful acoustic shock waves through the surface of the human body in such a way that the pressure increases at the target, the stones can be fragmented. Weak acoustic waves travel through the soft tissues without any damage as long as the pressure remains below a certain level. Focusing acoustic waves produces pressure above a given threshold to destroy targets.
Current commercial lithotripsy systems use X-ray or acoustic imaging techniques (ultrasound) to locate the target. X-ray imaging exposes the patient to ionizing radiation, and non-calcified stones which are most effectively treated with ESWL are not seen. Ultrasound is limited by poor image quality, including artifacts produced when imaging stones and stone fragments.
There are three common methods currently employed for shock wave generation: the spark generator, the piezo-electric array, and the electromagnetic acoustic generator.
Spark generators are used to create powerful electric sparks to generate shock waves. The poor focusing ability of the spark generator results in soft tissue damage around the stone. Another drawback of this technique stems from the rapid burn out rate of the electrodes of the spark generator, requiring replacement after each procedure. Piezo-electric generators build up shock waves by the displacement of a mosaic array of piezo-electric crystals. Even with large array size and good focusing, the shock waves achieve only a moderate pressure at the focal point, and therefore, are unable to break up larger calculi within a reasonable time.
Electromagnetic acoustic shock wave generators have been developed for implementation in this field. The early stage of this development is disclosed by Reichenberger et al. in their Siemens Research and Development Report, titled "Electromagnetic Acoustic Source for the Extracorporeal Generation of Shock Waves in Lithotripsy." (1986, vol. 15, 187-194). The electromagnetic acoustic source lithotripter includes a discharge capacitor as a power supply. An enameled copper wire slab coil is suspended by a ceramic support. The coil is separated from a metallic membrane by a thin insulating film. The coil and the conductive membrane act as the primary and secondary windings of a transformer. Upon application of a current to the coil via the discharge power supply, eddy currents are induced in the membrane which result in a repulsive force between the coil and the membrane. The membrane is thus caused to emit an acoustic pulse which is then focused on the target by an acoustic lens.